(1) Field of the Invention
The invention relates to a multi-stage overvoltage protection circuit, in particular for information technology systems, comprising at least one coarse-protection element and at least one fine-protection element, wherein the at least one fine-protection element can be activated by a triggering device depending on a threshold.
(2) Description of the Related Art
Overvoltage protection circuits consisting of multiple stages are described in the known prior art. Usually, a first stage is formed by a component capable of arresting high currents whilst having a comparatively high operating voltage. Such a component may be a gas discharge arrester. Although such a protection circuit can be used in a great nominal value range, it does not provide sufficient protection for information technology equipment such as computers, telephone systems or the like due to the high protection level. Therefore, there is usually provided an additional stage, in which a fine-protection element, namely a Z-diode, a TVS-diode or the like is provided, which is rated for the signal voltage so that a limitation as near as possible to the nominal voltage is guaranteed. At least one decoupling element is provided between the coarse-protection element and the fine-protection element, which serves to coordinate the behavior of the aforementioned stages.
An amplifier circuit comprising an overvoltage protection device is disclosed in DE 198 45 281 A1. The fine-protection element is a transistor short-circuiting the provided amplifier signal wires if a fixed threshold is exceeded. Specifically, a transistor protection circuit is connected downstream of the actual overvoltage arrester in the overvoltage interference signal direction, whose transistors are blocked during normal operation and are switched when an overvoltage occurs, by limiting the maximum acceptable voltage.
In the voltage-limiting device according to DE 10 2004 036 164 A1 at least one voltage input and one voltage output are provided. A first voltage-limiting unit is provided with at least one input and one output such that at least a first switch is provided in the first voltage-limiting unit between the input and the output. In addition, a voltage sensor is provided in the first voltage-limiting unit, which is configured to open the first switch when a voltage is present at the input of the first voltage-limiting unit that is greater than a predeterminable maximum voltage value.
EP 1 278 283 A2 describes a device for the protection of electronic components against overvoltages, comprising at least one transistor which is switchable by a trigger circuit upon reaching predetermined overvoltage values, thereby converting disturbing electrical energy to thermal energy. The fine-protection element used is a Z-diode.
PCT/WO 2004/006408 A1 discloses a protection circuit having an electronic decoupling element TBU which goes into a high-impedance state if a current value is exceeded, thereby limiting the flow of current to a terminal. The disadvantage of this teaching is that a voltage-limiting component has to be provided in the terminal in order to generate a sufficiently high flow of current in the event of an overvoltage.
In the protection unit for a modem according to PCT/WO 02/50973 A1 a fine-protection element is switched in by a thryristor. The thyristor is driven by the current flow to a terminal. The disadvantage of this circuit configuration is that this terminal, too, has to include a voltage-limiting component in order to generate a sufficient flow of current in the event of an overvoltage.